1. Field of the Invention
This invention is directed to a shaker table which permits a device to be screened under any desired vibrational conditions and, in particular, under laboratory or assembly line conditions.
2. Description of the Prior Art and Other Considerations
Such shaker tables, permitting a device or product to be shaken in the laboratory or in an assembly line, enables any defects, which may be a result of the manufacturing process, to be found. In this way, devices or products, which have defects caused by variations in fabrication processes or workmanship, may be screened out of the production line before being shipped to a customer.
In order to achieve the desired vibrational screening modes, various shaker table designs have been employed. Examples include those described in U.S. Pat. Nos. 4,181,025 through 4,181,029 and the references cited or discussed therein. An exemplary shaker table comprises a platform, to which the device to be screened is secured, and vibration drivers for the platform. One problem with a simple shaker table is that damping is low so that, when a resonance point is reached within the frequency range of the screen, the amplitude rises to a sharp and potentially destructive peak. The result is that the device is screened at a high amplitude over the narrow range of resonance and a much lower amplitude over the remainder of the frequency band. Unless the resonance points of the screen excite the resonances of the device without over-stress, the vibration screen will damage the device or will, by necessity, be run at such a low overall acceleration level as to be ineffective. It has become increasingly evident throughout the industry that multi-axial (three to six simultaneous degrees-of-freedom) random vibration screening is more efficient both in the time it takes to complete a screen and in the number and type of defects detected by the screen. Because all vibrational axes of interest may be screened simultaneously, a multi-axis screen sequence usually takes one-third the time of that required by a single axis system. In addition, the multi-axial excitation of the device tends to cause a higher number of intermittent defects to be detected because of the increased number of acceleration vectors experienced by the device's components. Thus, there is need for a vibration table which translates multiple vibration pulses into a quasi-random, multi-degree-of-freedom, acceleration spectrum which does not have significant individual resonance peaks, but is highly damped and has a plurality of highly damped resonance points so that the table amplitude is substantially constant over the frequency range of the shaker table.